Scanning electron microscopes are commonly used for observation and analysis of matter. Scanning electron microscopes have found broad application in the transistor industry to show voltage distribution in such devices. Other uses include industrial quality control and a broad range of industrial and biological research.
A scanning electron microscope typically includes a vacuum chamber, an electron optical system for generating and focusing an electron beam (referred to also as the primary electron beam) at a specimen, a deflection system for moving the beam across a specimen in a predetermined raster pattern, a detector system for detecting phenomena from the specimen caused by the impinging primary electron beam, and a display system.
When the primary electron beam strikes the specimen, a complex response is generated, including both short-lived and long-lived phenomena. The short-lived phenomena include secondary electrons (low energy), back-scattered electrons (high energy), x-ray characteristics of the specimen, "white x-rays", light (cathode luminescence), absorbed electrons, transmitted electrons, and auger process electrons (low energy). Detectors for each of the above phenomena are well known in the art.
In a conventional scanning electron microscope, typically at least one of the signals identified above is detected, amplified and displayed on a CRT (cathode ray tube). The amplitude of the signal is used to modulate the intensity of the beam of the CRT. The beam of the CRT is deflected in a raster pattern which corresponds to and is synchronized with the scanning primary electron beam of the SEM. A black and white image of the specimen is thereby presented to the operator of the microscope. The image thus created may be set to contain three types of information at each point on the CRT, two position vectors which identify the location of the primary electron beam on the specimen, and one brightness or intensity level vector which contains information about the specimen. The intensity level is usually derived from the secondary electron emission, which contains topographical (slope) information. The information thus presented is in a form which is readily accepted by the human operator who can rapidly assimilate the information. One or more of the many signals which are induced by the primary electron beam can be displayed in a conventional scanning electron microscope system at any one time.
Each of the various responses of the specimen to the primary electron beam includes unique information about the specimen. For example, the intensity of the secondary electron emission contains information about the slope of the specimen surface, with respect to the primary electron beam. This information can be used to generate an image of the specimen surface. As another example, the back-scattered (high energy) electron signal contains information about the atomic number of the specimen, and thus can be used to provide a profile having an intensity which is representative of the chemical makeup rather than the shape of the specimen under examination. In a conventional scanning electron microscope with display, the operator can observe any one or more than one of these images at a time.
Color synthesizers have been proposed in the prior art to enhance the display image of scanning electron microscopes. For the most part, these color imaging schemes have not increased the information content of the image but have been utilized merely to present a more aesthetically pleasing picture.
Color pictures have been produced photographically by means of multiple exposures of film through appropriately colored filters. One prior system utilizes three separate x-rays, each representative of a different element, which are used to modulate the three electron guns of a color kinescope. That is, each x-ray detector is associated with a different color and these elements are then displayed concurrently and in color. The resultant image defines the distribution of the elements in the specimen since each one is represented by a different color.
In addition, colored images are constructed in which different regions of the image are colored in accordance with a color coding scheme, in such a fashion that variations in a physical variable are represented by different colors in the image. While such colored images convey information in a fashion which can be readily assimilated, the information content is generally no greater than would be conveyed had the colors not been used and the image simply coded monochromatically such as by different shading patterns. This is in contrast to the images normally perceived by the eye where, because of the added mixed light impressions perceived, a variety of lighting effects can be differentiated, which impressions can hardly be adequately represented monochromatically. This arises because a colored image as perceived by the eye can be considered as being made up of three simultaneously perceived images, each of a separate primary color, and the eye is capable of differentiating regions in the image as to which of the relative proportions of the three primary colors vary.
U.S. Pat. Nos. 3,628,014, 4,041,311 and 4,560,872 propose techniques which generate color images from the information available in a scanning electron microscope and display the images on a color CRT. The images are continuous in hue and cover the entire color range. Such techniques, of course, generate color images in real time as distinguished from photographic images. However, these techniques are not useful for producing color images at the low voltages typically used during examination of biological specimens. Moreover, the images produced, although satisfactory, do not provide a substantial amount of information about the specimen.